1. Field of the Disclosure
In one aspect, this disclosure generally relates methods and apparatuses for estimating reservoir connectivity.
2. Background of the Art
In exploration, development, and monitoring related to hydrocarbon production, it is important to make accurate measurements of geologic formations. The geologic formations below the surface of the earth may contain reservoirs of oil and gas. The geologic formations may include formation layers and various structures. In a quest for oil and gas, it is important to know about the location and composition of the formation layers and the various structures. In particular, it is important to know about the geologic formations with a high degree of accuracy so that resources are not wasted. Measuring properties of the geologic formations provides information that can be useful for locating the reservoirs of oil and gas. Generally, the oil and gas are retrieved by drilling boreholes into the subsurface of the earth. The boreholes also provide access for taking measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the borehole. In one embodiment, a “logging instrument” is lowered on the end of a wireline into the borehole. The logging instrument sends data via the wireline to the surface for recording. Output from the logging instrument comes in various forms and may be referred to as a “log.” Many types of measurements are made to obtain information about the geologic formations. Some examples of the measurements include gamma-gamma density logs, gamma ray logs, nuclear magnetic resonance logs, neutron logs, resistivity logs, and sonic or acoustic logs.
Typically, a common factor among the logs is that a borehole depth is associated with the measurements. For example, each time a resistivity measurement is made, the measurement is associated with the borehole depth at which the measurement was made. In general, many logs of various measurements are analyzed to make an accurate assessment of the geologic formations. The various measurements may be viewed side-by-side to form a composite picture of the geologic formations. Therefore, it is important to have accurate knowledge of the borehole depth and orientation of the logging tool when each measurement is taken.
To develop and manage hydrocarbon reservoirs it is critical to estimate reservoir connectivity. Traditionally formation evaluation measurements are made versus measured depth and then converted to TVD using well inclination data obtained from survey data. Ultimately when comparing fluid contacts from one well with another, cumulative errors in the measured depth that propagate to the computed TVD make it difficult to determine if two contact levels measured in different wells are at the same depth. The inability to determine whether the contact is at the same depth means it is difficult to determine whether the reservoir is compartmentalized and whether additional wells are needed to provide pressure support and to efficiently produce the hydrocarbons. Further, the contact depth (TVD) may be needed to compute the volume of hydrocarbons in a structure, where the structure may be known from a geologic model or surface seismic data. Horizontal offsets of the borehole may also lead to errors in measuring the borehole depth. It is, therefore, important to know the “true vertical depth” of the logging instrument. The horizontal offsets are not relevant to the true vertical depth.